This is a question an old trivial pursuit card posed. My friends and I argued about it. The supposed answer was that cold water was heavier. That sounds absurd to me, though I don’t know anything about physics. Hot water has molecules that dance faster, higher energy that should interact with gravity, lower density… it makes intuitive sense to me that hot water should be heavier.
It definitely asked about weight rather than mass, by the way. I’m sure that the answer is “this question is based on a false premise and experimental results would be the opposite of theoretical calculations” because that’s how these things tend to work out. Google the thing and you’ll see equal claims for either temperature weighing more.
Well, if the question was worded something like “Which is heavier, a pound of hot water or a pound of cold water?”, I would suppose that the supposed answer isn’t right.
Hot water is less dense than cold water.
Therefore, a given volume of hot water will weigh less than the same volume of cold water. The mass of the energy in the water is insignificant in this case.
Colder water is denser than hoter water, until it hits the freezing point, and becomes less dense. Or the boiling point. So I think that one could be argued.
Of what interactions between energy and gravity do you speak ?
Well anyway the idea is that its a tiny tiny tiny tiny (… etc…) effect.
The change in density is real.
For example, its most dense at 3 degrees C… Just below freezing. This means that you never end up with a supercooled lake/ocean … imagine if you dove down and when you touched the water it turned to ice ? but if it was really close to freezing it would float toward the surface (back into warm water perhaps, where it warms up again… but if its all really cold, up to the top.), and therefore freezing always occurs at the top.
Generally, fluids expand when heated because the vibrating molecules are taking up more space. That’s true of solids too. Why you think that hot molecules should interact more with gravity is quite beyond me, but it’s a hypothesis that exists in flat defiance of how, say, thermometers and hot-air balloons behave or how water starts to boil in a pot, or any other instance of convection.
What is odd about water is that it expands again between 4 degrees Celsius and freezing.
It is a fascinating point that if water were not this odd, life would not have occurred as we know it. Most liquids on cooling solidify to the bottom of a vessel. Silcion and Water are unusual in that there atomic structure means that just at the point that they solidify, their structure changes to make their solid lighter than their liquid form.
If water was like virtually every other liquid, life could not have easily survived on earth as it has. There would be some life (we know some organisms can still exist in frozen conditions), but an easy transition to more complex beings without the ability for their predecessors to survive at the bottom of oceans and rivers would have made the survival of aquatic and amphibious animals less likely. We might be a single celled universe!
Can you explain further the logical connection between these two sentences? Why does the fact that water is at its most dense just above (not below) freezing mean that water in a lake or ocean can’t be supercooled?
Density equals mass over volume; so volume equals mass divided by density, and mass equals density times volume. You want to find the mass of a given volume of water at two temperatures. At 4ºC water has a density of (essentially) 1.0 g/mL. At 99ºC, density is about 0.96 g/mL. So if the volumes are the same, the cold water would have more mass for that volume.
I’m not sure what he means by supercooled, but water’s unusual change in density around it’s freezing point prevents large bodies of water from freezing solid. First of all, the layer of ice insulates the underlying water from cold outside air, and secondly, the change of density near freezing messes up convection currents, so that the body doesn’t tend to have the coldest water at the bottom, like it would if it were made of a “normal” liquid.
I don’t like this question because it’s not clear to me. If I have a sealed container with a given quantity of water, and then I add heat to it, then certainly the warmer water is heavier (if ever only so slightly) due to relativistic effects. If the question was really just “what weighs more, warm or cold water?” then I think it’s unclear what they are asking. If they are talking about a given volume of water, then of course the density is important and the colder water is heavier. If they are talking about a given amount of water (number of molecules), then warmer is heavier due to relativity.
Can you explain the relativistic effects? If we assume one mole of water molecules in a sealed container, then we have 18.01528 grams of water regardless of the state.
E = MC Squared implies that Mass transforms to Energy as the Mass approaches C the speed of light. A Mass at rest can absorb Energy but that is only going to rip the atoms apart.
No, that’s all kinds of wrong. The relativistic formula for kinetic energy is not E = mc^2, and it happens to include the object’s actual speed: Kinetic energy - Wikipedia
E=mc^2 implies that any increase in energy is an increase in mass, and vice versa. That’s how nuclear fusion and fission is fueled, a small loss in mass corresponding to a large release of energy. It works the other way around as well, but for the comparably tiny amounts of energy involved in everyday prosesses the increase in mass is insignificant.
One of my favourite questions on the topic is “Is there a change in mass when I add energy to a plate by lifting it to a higher shelf?”